The Mar phenotype in E. coli is attributed largely to the action of MarA, the expression of which is regulated by MarR (Alekshun, M. N. supra (1997)). MarA is a transcription factor that autoactivates expression of the marRAB operon and regulates the expression of a global network of more than 60 chromosomal genes (Martin, R. G. et al. J. Bact. 178, 2216–2223 (1996); Barbosa, T. M. & Levy, S. B. J. Bact. 182, 3467–3474 (2000)). Mar mutants in isolates of clinical origin have now been identified (Maneewannakul, K. & Levy, S. B. Antimicrob. Agents Chemother. 40, 1695–1698 (1996); Oethinger, M. et al. Antimicrob. Agents Chemother. 42, 2089–2094 (1998); Linde, H. J. et al. Antimicrob. Agents Chemother. 44, 1865–1868 (2000); Ziha-Zarifi, I., et al. Antimicrob. Agents Chemother. 43, 287–291 (1999); Koutsolioutsou, A et al. Antimicrob. Agents Chemother. 45, 38–43 (2001)). Constitutive overexpression of MarA or a MarA homolog in many of these strains is a key contributor to the maintenance of the resistance phenotype, particularly with respect to the fluoroquinolones, and recent studies have documented the selection of Mar mutants, bearing mutations in MarR, MexR, or other homologous loci, in E. coli, Pseudomonas aeruginosa, and other organisms during antimicrobial chemotherapy (Oethinger, M supra; Linde, H. J. et al.; supra; Ziha-Zarifi, I. et al. supra; Kern, W. V., et al. Antimicrob. Agents Chemother. 44, 814–820 (2000)).
MarR is a regulator of multiple antibiotic resistance in Escherichia coli. It is the prototypic member of a family of regulatory proteins found in the Bacteria and the Archae that play important roles in the development of antibiotic resistance, a global health problem. In the absence of an appropriate stimulus, MarR negatively regulates expression of the marRAB operon (Cohen, S. P., et al. 1993. J. Bacteriol. 175: 1484–1492.; Martin, R. G. and Rosner, J. L. 1995. Proc. Natl. Acad. Sci. 92: 5456–5460; Seoane, A. S. and Levy, S. B. 1995. J. Bacteriol. 177: 3414–3419., 1995). DNA footprinting experiments suggest that MarR dimerizes at two locations, sites I and II, within the mar operator (marO) (Martin and Rosner, 1995, supra). Site I is positioned among the −35 and −10 hexamers and site II spans the putative MarR ribosome binding site (reviewed in Alekshun, M. N. and Levy, S. B. 1997. Antimicrob. Agents Chemother. 10: 2067–2075).
MarR is a member of a newly recognized family of regulatory proteins (Alekshun, M. N. and Levy, S. B. 1997. Antimicrob. Agents Chemother. 10: 2067–2075. Sulavik, M. C., et al. 1995. Mol. Med. 1: 436–446) and many functional homologues have been identified in a variety of important human pathogens and have been found to regulate a variety of different processes. For example, some MarR homologues have been found to control expression of multiple antibiotic resistance operons, some regulate tissue-specific adhesive properties, some control expression of a cryptic hemolysin, some regulate protease production, and some regulate sporulation. Proteins of the MarR family control an assortment of biological functions including resistance to multiple antibiotics, organic solvents, household disinfectants, and oxidative stress agents, collectively termed the multiple antibiotic resistance (Mar) phenotype (Alekshun, M. N. & Levy, S. B. Trends Microbiol. 7, 410–413 (1999)). These proteins also regulate the synthesis of pathogenic factors in microbes that infect humans and plants (Miller, P. F. & Sulavik, M. C. Mol. Microbiol. 21, 441–448 (1996)). Insight into the three dimensional structure of MarR family proteins would be of great value in designing drugs that interact with this family of proteins and modulate MarR function, for example, antibiotic resistance and virulence.